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ECGD 4122 – Foundation Engineering Lecture 2 Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2 nd Semester 2008/2009

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Revision of Soil Mechanics Soil Composition Soil Classification Groundwater Stress (Total vs. Effective) Settlement Strength 2

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Soil: A 3-Phase Material Solid Water Air 3

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The Mineral Skeleton Volume Solid Particles Voids (air or water) 4

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Three Phase Diagram Solid Air Water Mineral Skeleton Idealization: Three Phase Diagram 5

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Fully Saturated Soils Fully Saturated Water Solid Mineral Skeleton 6

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Dry Soils Mineral Skeleton Dry Soil Air Solid 7

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Partially Saturated Soils Solid Air Water Mineral SkeletonPartly Saturated Soils 8

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Three Phase System Volume Weight Solid Air Water WTWT WsWs WwWw W a ~0 VsVs VaVa VwVw VvVv VTVT 9

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Weight Relationships Weight Components: Weight of Solids = W s Weight of Water = W w Weight of Air ~ 0 10

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Volumetric Relationships Volume Components: Volume of Solids = V s Volume of Water = V w Volume of Air = V a Volume of Voids = V a + V w = V v 11

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Volumetric Relationships Volume Components: Volume of Solids = V s Volume of Water = V w Volume of Air = V a Volume of Voids = V a + V w = V v 12

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Specific Gravity Unit weight of Water, w w = 1.0 g/cm 3 (strictly accurate at 4° C) w = 62.4 pcf w = 9.81 kN/m 3 13

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Specific Gravity, G s Iron7.86 Aluminum Lead11.34 Mercury13.55 Granite2.69 Marble2.69 Quartz2.60 Feldspar

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Specific Gravity, G s 15

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Example: Volumetric Ratios Determine void ratio, porosity and degree of saturation of a soil core sample Data: Weight of soil sample = 1013g Vol. of soil sample = 585.0cm 3 Specific Gravity, G s = 2.65 Dry weight of soil = 904.0g 16

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Solid Air Water W a ~0 Volumes Weights g 585.0cm g s = g 341.1cm cm cm cm 3 W =1.00 Example 17

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585.0cm 3 Solid Air Water Volumes s = cm cm cm cm 3 W =1.00 Example 18

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Soil Unit weight (lb/ft 3 or kN/m 3 ) Bulk (or Total) Unit weight = W T / V T Dry unit weight d = W s / V T Buoyant (submerged) unit weight b = - w 19

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Typical Unit weights 20

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Fine-Grained vs. Coarse-Grained Soils U.S. Standard Sieve - No inches mm “ No. 200 ” means... 21

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Sieve Analysis ( Mechanical Analysis) This procedure is suitable for coarse grained soils e.g. No.10 sieve …. has 10 apertures per linear inch 22

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Hydrometer Analysis Also called Sedimentation Analysis Stoke’s Law 23

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Grain Size Distribution Curves 24

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Soil Plasticity Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity. Albert Atterberg, Swedish Soil Scientist ( ) …..series of tests for evaluating soil plasticity Arthur Casagrande adopted these tests for geotechnical engineering purposes 25

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Consistency of fine-grained soil varies in proportion to the water content Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid semi-solid plastic liquid Plasticity Index (cheese) (pea soup) (pea nut butter) (hard candy) 26

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Liquid Limit (LL or w L ) Empirical Definition The moisture content at which a 2 mm- wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device 27

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Engineering Characterization of Soils Soil Properties that Control its Engineering Behavior Particle Size Particle/Grain Size Distribution Particle Shape Soil Plasticity fine-grainedcoarse-grained 28

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Clay Morphology Scanning Electron Microscope (SEM) Shows that clay particles consist of stacks of plate-like layers 29

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Soil Consistency Limits Albert Atterberg ( ) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911) Arthur Casagrande ( ) …… A dopted these tests for geotechnical engineering purposes 30

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Arthur Casagrande ( ) Joined Karl Terzaghi at MIT in 1926 as his graduate student Research project funded by Bureau of Public Roads After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard Soil Plasticity and Soil Classification (1932) 31

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Casagrande Apparatus 32

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Casagrande Apparatus 33

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Casagrande Apparatus 34

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Liquid Limit Determination 35

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The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches Plastic Limit (PL, w P ) 36

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Plastic Limit (PL, w P ) 37

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Plasticity Index ( PI, I P ) PI = LL – PL or I P =w L -w P Note: These are water contents, but the percentage sign is not typically shown. 38

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Plasticity Chart 39

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USCS Classification Chart 40

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USCS Classification Chart 41

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Plasticity Chart 42

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Groundwater U = porewater pressure = w Z w 43

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Stresses in Soil Masses Area = A = P/A X X Soil Unit P Assume the soil is fully saturated, all voids are filled with water. 44

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Effective Stress From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil. Therefore, we may define effective stress = total stress minus pore pressure ′ = - u where, ′ = effective stress = total stress u = pore pressure 45

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Effective Stress ′ = - u The effective stress is the force carried by the soil skeleton divided by the total area of the surface. The effective stress controls certain aspects of soil behavior, notably, compression & strength. 46

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Effective Stress Calculations ′ z = i H i - u where, H = layer thickness sat = saturated unit weight U = pore pressure = w Z w When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress. 47

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Geostatic Stresses 48

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Compressibility & Settlement Settlement requirements often control the design of foundations This chapter provides a general overview of principles involved in settlement analysis The subject will be dealt with in greater detail in Chapter 7. 49

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Increase in Vertical Effective Stress Due to a Placement of a fill Due to an external load 50

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Voids Solids H V v = eV s VsVs c c e e V v = (e - e)V s VsVs Solids z′z′ z′z′ z0 ′ } z f ′ Before After Consolidation 51

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u0u0 00 Before Loading Point, P 52

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u0+uu0+u 0 + Immediately After Loading Point, P 53

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0 + u0+uu0+u u0u0 Shortly after Loading No settlement Long after Loading Settlement Complete 54

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Settlement Distortion Settlement (Immediate) Consolidation (Time Dependent) Secondary Compression Time Settlement 55

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Laboratory Consolidation Test 56

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Consolidation Test 57

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Test Results 58

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Consolidation Plot 59

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Test Results Idealized Data 60

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Compression Index and Recompression Index 61

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Compression Ratio and Recompression Ratio 62

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Normally and Over-Consolidated Soils ….. Normally consolidated ….. Over consolidated ….. Under consolidated 63

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Over-Consolidation Margin & Over- consolidation Ratio ….. Over-consolidation Margin ….. Over consolidation ratio 64

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Typical Range of OC Margins 65

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Compressibility of Sand and Gravels (Table 3.7) 66

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Example 3 67

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Settlement Predictions N.C. Clays 68

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Settlement Predictions O.C. Clays…… Case I 69

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Settlement Predictions O.C. Clays…… Case II 70

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Example 4 71

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Example 4 72

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Example 5 73

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Example 5 74

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Slope Failure in Soils Failure due to inadequate strength at shear interface 75

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Shear Failure in Soils 76

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Shear Failure in Soils 77

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Bearing Capacity Failure 78

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Transcosna Grain Elevator Canada (Oct. 18, 1913) West side of foundation sank 24-ft 79

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Shear Strength of Soils Soil derives its shear strength from two sources: Cohesion between particles (stress independent component) Cementation between sand grains Electrostatic attraction between clay particles Frictional resistance between particles (stress dependent component) 80

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Shear Strength of Soils; Cohesion Dry sand with no cementation Dry sand with some cementation Soft clay Stiff clay 81

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Shear Strength of Soils; Internal Friction 82

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Shear Strength, S Normal Stress, = C = Mohr-Coulomb Failure Criterion 83

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Shear Strength is controlled by Effective Stress, ' Potential Failure Surface Slope Surface 84

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Mohr-Coulomb Failure Criterion 85

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Typical Values 86

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Effect of Pore Water on Shear Strength Pore water pressure Total Stress, versus Effective Stress, Shear Strength in terms of effective stress 87

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Moist beach sand has apparent cohesion Negative pore water pressures Apparent Cohesion 88

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Measuring Shear Strength Laboratory Direct shear test Unconfined compression test Triaxial compression test Field Vane shear test 89

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Direct Shear Test ASTM D-3080; AASHTO T

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Direct Shear Test 91

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Direct Shear Test 92

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Direct Shear Test Device 93

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Direct Shear Test Device 94

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Direct Shear Test Data Shear stress 95

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Direct Shear Test Data Volume change 96

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Peak vs. Ultimate Strength 97

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Example: Direct Shear Test Given: A direct shear test conducted on a soil sample yielded the following results: Normal Stress, (psi) Max. Shear Stress, S (psi) Required: Determine shear strength parameters of the soil 98

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Example 6 99

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Drained versus Undrained Conditions …. Before loading After loading 100

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Drained versus Undrained Conditions …. Before loading After loading 101

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Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drain Sands drain fast; therefore under most loading conditions drained conditions exist in sands Exceptions: pile driving, earthquake loading in fine sands Soil Shear Strength under Drained and Undrained Conditions …. 104

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In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quickly Therefore, in clays the short-term shear strength may correspond to undrained conditions Even in clays, long-term shear strength is estimated assuming drained conditions Soil Shear Strength under Drained and Undrained Conditions …. 105

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Shear Strength in terms of Total Stress Shear Strength in terms of effective stress Shear strength in terms of total stress u at hydrostatic value 106

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Long-term Stability Potential Failure Surface Slope Surface 107

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Short-term Stability Potential Failure Surface Slope Surface 108

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Shear Strength in terms of Total Stress; = 0 condition Shear strength in terms of total stress For cohesive soils under saturated conditions, =

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Shear Strength, S Normal Stress, C = 0 Mohr-Coulomb Failure Criterion 110

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Mohr’s Circles 3 =0 11 Direct Shear Uniaxial Compression 111

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Mohr’s Circles 3 =0 11 Uniaxial Compression 11 Horiz. plane Max. shear plane 112

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Mohr’s Circles 3 =0 11 Uniaxial Compression 113

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Unconfined Compression Test ASTM D-2166; AASHTO 208 For clay soils Cylindrical specimen No confining stress (i.e. 3 = 0) Axial stress = 1 3 = 0 11 114

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Unconfined Compression Test Data 115

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Unconfined Compression Test 116

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Example: Unconfined Compression Test Given: An unconfined compression test conducted on a soil sample yielded the results shown in the table. Required: Determine undrained shear strength, S u of the soil 117

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Example: Unconfined Compression Test 118

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Example: Unconfined Compression Test q u = 43.45psi=6257 psf S u = 21.7psi = 3128 psf 119

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Triaxial Compression Test Unconfined compression test is used when = 0 assumption is valid Triaxial compression is a more generalized version Sample is first compressed isotropically and then sheared by axial loading 11 33 120

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Triaxial Compression Test Load applied in 2 stages confining pressure, 3 dev. stress, = 1 - 3 11 33 33 121

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Triaxial Compression Test 122

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Triaxial Compression Test 123

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Triaxial Compression Test for Undisturbed Soils 124

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Drainage during Triaxial Compression Test 125

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Triaxial Compression Tests Unconsolidated Undrained (UU- Test); Also called “ Undrained ” Test Consolidated Undrained Test (CU- Test) Consolidated Drained (CD-Test); Also called “ Drained Test ” 126

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Triaxial Compression Tests ASTM Standards ASTM D2850: Unconsolidated Undrained Triaxial Test for Cohesive Soils ASTM D4767: Consolidated Undrained Triaxial Test for Cohesive Soils 127

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Triaxial Compression Tests AASHTO Standards AASHTO T-296: Unconsolidated Undrained Triaxial Test for Cohesive Soils AASHTO T-297 : Consolidated Undrained Triaxial Test for Cohesive Soils 128

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Consolidated Undrained Triaxial Test for Undisturbed Soils 129

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